Tips on How to Fix a Flat Roof in the Winter

During the winter season, our company receives a number of requests inquiring about how to fix a flat roof in the winter. Perhaps one of the first things to understand for some people is why a flat roof may leak in the absence of rainy weather. Rain does occur during the winter season usually when there is an upward temperature swing, and freezing rain develops when an opposing cold front event is introduced.

But what about leaks on flat roofs which happen seemingly when there has been no precipitation for days? To answer the question of why winter leaks for all of the above described weather variations; one must consider a few elements.

The first to consider is the age and condition of the particular flat roof. Flat roofs are generally but not necessarily, a multi-ply waterproofing system; unlike a sloping roof assembly which is considered water shedding.

On residential flat roofs; they are most often either a four ply built up roof system incorporating hot asphalt and a gravel finished coating, or a two ply modified bitumen system that has been heat welded or cold asphalt applied. Commercial and industrial roofs may also utilize single-ply waterproofing system technology as their specified assemblies.

Each type of flat roofing waterproofing system has its own unique characteristics which can provide clues for the attending roofing contractor in relation to potential breach or failure points. In very general terms if a roof does work, and then all of a sudden does not work; there is a physical reason for the leaking circumstance.

Some of the most common “winter leaks” on flat roofs in winter can result from roof drains, whether internal or external types. If a drain freezes up and water cannot exit the roof; it may back up under nearby metal flashing details skylight curbs, etc. The first preventative winter maintenance tip is therefore to ensure roof drains and associated piping are clear of debris or blockage and to check that roof drain connections below the roof line are intact. A rusted connection, even with pin-hole sized corrosion can result in leaks and in colder weather may cause a drain or piping to burst.

Next to consider is the integrity of the waterproofing membrane. If the system is older and nearing the end of its usable life-cycle (15-25 years on average); residual water which may freeze into the surface of the membrane will form sheet ice. The ice and the roof itself can expand and will contract with temperature change (termed thermal shock or thermal movement/ a process of thermal expansion and contraction); with a possible result of the membrane splitting open at a seam, in the field of the roof, or at a protrusion point. The next tip is to ensure that an older water-proofing membrane is either replaced or repaired in a competent manner to extend its usable life. Where expensive interior building assets are concerned, the wisest approach is to replace the assembly so it becomes effectively ”bullet proof”. This is the best defense against failure from climate extremes and is typically a preferred scenario by insurers.

Interesting to note is that in Southern Ontario, we experience at least seventy freeze-thaw cycles per winter. Dramatic temperature shifts upward and downward are known to stress many building components aside from roof waterproofing materials.

Many winter roof leaks on flat roofs will demonstrate themselves in “appliance areas” located on the roof. These include but are not limited to, pipes, insulated chimneys, masonry chimneys, skylight curbs and units, HVAC equipment, ducting, perimeter metal flashings, door sills, and thresholds, roof to slope junctions, roof to sided wall junctions, ventilators, drains, guttering, etc.

As a result; if one develops signs of any such leaks. It is wise to measure the internal damage coordinates and check them in relationship to appliance locations on the roof which may be in close proximity to the interior leak points. To note is that water may travel underneath the roof’s substrate so in the event of a leak, it is wise to poke a hole in the ceiling and to contain any water ingress from spreading across the ceiling, and by protecting the flooring beneath with a tarp and utilizing a large garbage pail to catch the water. Of course, contacting a professional roofer is the next step under such a circumstance.

Some “winter” flat roof leaks are not actually flat roof leaks at all. Severe temperature changes in winter can prompt condensation to develop beneath a flat roof and mimic that the roof is actually leaking. One of the first questions one should consider is; has this roof leaked during milder weather and under rain conditions? If the answer is no, then it is likely a circumstantial leak event. There tends to be a surge in calls for service when the temperature is extremely cold and people experience these phenomenon.

Condensation leaks will appear often as brown moisture stains around skylight wells and to ceiling junctions, surrounding pot lights located beneath flat roofs, where ceilings may adjoin a wall, fireplace, or room changes as some possible examples.

The causes can be many, or multiple issues which result in “condensation leaks”. These can include, breaches in or lack of, a sufficient interior vapour barrier, inadequate insulation beneath the roof deck or skylight well, insufficient insulation and a vapour retarder layer above the roof deck, heat loss, air leakage, etc.

Other winter leak origins can result from accumulated snow volume where for example snow may creep up under perimeter metal flashings and enter where membrane flashings terminate beneath the metal work or where from capillary action; snow may get sucked up under a skylight frame.

In order to stop winter leaks on flat roofs, preparation is required. The first step is to remove excess snow and ice; which is also a preventative action particularly on commercial steel decked roofs where excessive weight build up can be a safety hazard leading to collapse.

The next step involves proper roof safety to secure attendant workers completing the project and to prevent property owner liability. Isolated leaks such as at an appliance locations can then be repaired.

If the system is older, repairs may not represent the best investment but may serve to extend the inevitable roof replacement until better weather.

Flat roofs can be replaced in the winter and tear off may become easier due to colder temperature conditions. There is certainly less risk of damage to gardens and lawns. The risk of rain is also greatly reduced. The important thing to remember is that certain flat roofing products may not be suited to winter installation. Single ply assemblies being one, as they are reliant on adhesives that need to bond in above zero temperatures.

Factually flat roofing is completed all year round in Canada, however; one must work in conditions that are suitable for particular product reliability and installer comfort and safety.

To learn more about solving winter roof leaks, commercial /industrial or residential flat roofing, and other related building envelope services; visit: or view our work on YOUTUBE™